Time Division Multiplex (TDM) systems typically include a TDM bus and a plurality of stations connected to the bus. One such TDM data communication system is a Metropolitan Area Network (MAN) which utilizes a standardized (IEEE 802.6 protocol) Distributed Queuing Dual Bus (DQDB) arrangement. The DQDB arrangement consists of two unidirectional buses with a plurality of stations connected between the buses. Each station sends and receives data in time slots on the buses. The use of a time slot by a station makes that slot unavailable for use by other "downstream" stations on each bus.
To ensure that "upstream" stations do not unfairly monopolize the idle time slots, each station includes request (REQ) and countdown (CD) counter. When a station is idle, the REQ counter connects to a first bus and increments its count when requests for idle slots are received from downstream stations and decrements its count when empty time slots are received over the second bus. Thus, at any point in time, the REQ counter at each station indicates the unfulfilled requests for idle time slots from downstream stations. When a station becomes "active" and requests to transmit information the REQ count is transferred to the CD counter. The active station knows that its turn to transmit occurs when the CD counter reaches zero.
The present IEEE 802.6 protocol also includes a time slot reuse or destination release capability which enables a destination station to return a busy time slot to an idle time slot once it has reached that destination station. However, this time slot reuse capability requires that a new field, negative request (NR) field, in the header of each time slot be used to decrement the REQ counter to maintain the proper count of outstanding requests for idle time slots. Undesirably, the utilization of an NR field in the header depletes the protocol of valuable fields which may be needed for future features now being developed.